A ladder structure is deemed to be an ideal molecular structure for heat-resistant polymers. Carbocyclic or heterocyclic ladder polymers heretofore proposed include polyacene and polyperynaphthalene. It is also known that polyimide having partially incorporated thereinto a ladder structure exhibits improved performance properties. The reaction for introducing a ladder structure into these carbon series polymers can be achieved only under extremely severe conditions, while polysiloxanes having a ladder structure can be synthesized under relatively mild conditions because the synthesis consists in condensation based on an equilibrium reaction and also because condensation proceeds via a cyclic compound, and therefore, have been given study for years.
However, the chief subject of the conventional study on polysiloxane has been centered at ladder silicone having a phenyl type substituent at the side chain thereof, because it is easy to obtain this type of ladder silicone with high structural regularity and the product has excellent storage stability. For example, Nakahama, et al. synthesized ladder silicone having a phenyl group and a methacryloxypropyl group at the side chain thereof and incorporated this structure into a polystyrene molecular structure as disclosed in Polymer Preprints Japan, Vol. 29, No. 1, p. 73 (1980).
Nakahama, et al. mentioned in their report that synthesis of ladder silicone having both a methyl group and a reactive group was impossible due to occurrence of gelation. In general, it is admittedly difficult to synthesize ladder silicone having a methyl group at the side chain without being accompanied by gelation on account of its extremely high reactivity. Even if it can be synthesized, the product is unstable that it cannot be stored either as frozen or as dissolved in low concentrations.
The present inventors have keenly realized based on analysis of various data that it is ladder silicone having a methyl group at the side chain thereof that can draw out the best characteristics of ladder silicone, such as hardness, heat resistance, weather resistance, and light resistance.
On the other hand, it is known in the art that characteristics of organic polymers can be improved by incorporating a polysiloxane structure into the main chain or side chain thereof in various ways. For example, JP-A-60-231720 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process for preparing a polysiloxane-grafted copolymer with a polysiloxane structure in the side chain thereof by radical copolymerization of a siloxane-containing monomer and an ethylenically unsaturated monomer or a diene monomer. It is mentioned here that the resulting polysiloxane-grafted copolymer is excellent in surface properties, such as water repellency, stain resistance and release properties, and durability.
However, if the polysiloxane content in a polymer is increased aiming at further improvement in durability, the resulting polysiloxane-grafted copolymer would have reduced hardness, getting liable to scratches, and also reduced stain resistance because polysiloxane itself has softness.
JP-A-62-275132 proposes a process for producing a vinyl polymer having incorporated thereinto a siloxane side chain having two or more functional groups, which comprises copolymerizing a vinyl monomer with a polysiloxane macromonomer containing a polymethylsilsesquioxane structure (i.e., a ladder structure) and having two or more functional groups selected from a hydroxyl group and an alkoxy group. It is mentioned here that the polysiloxane-containing vinyl polymer is excellent in crosslinkability and compatibility with other resins as well as other characteristics, such as weather resistance, light resistance, water resistance, and stain resistance. This polysiloxane-containing polymer has high hardness despite of a high polysiloxane content since the polysiloxane structure thereof has a ladder structure and, in addition, the crosslinking density is increased by the functional groups. However, such a vinyl polymer having incorporated thereinto a siloxane side chain has also been demanded to have further improved durability and hardness.
As stated above, a polymethylsilsesquioxane having, in its side chains, a methyl group in a given or higher proportion and an aromatic group in a small proportion and also having, in its terminals or side chains, a functional group, such as a hydroxyl group, an alkoxy group, etc., is extremely liable to gelation due to the high reactivity of the functional group, as reported by Nakahama, et al. (Polymer Preprints, Japan, Vol. 29, No. 1, p. 73 (1980)).
Thus, it is very difficult to avoid gelation in synthesizing a polysiloxane having, in its side chains, a given or larger number of a methyl group and also having, in its terminals or side chain, functional a given or larger number of functional groups, such as a hydroxyl group, an alkoxy group, etc. Moreover, a polymer having such a polysiloxane structure incorporated into the molecule thereof has poor storage stability.
The above-mentioned polysiloxane-containing vinyl polymer as described in JP-A-62-275132 has high hardness owing to its high crosslink density but tends to lose crack-bridging properties on a substrate on which it is applied when the substrate undergoes great changes in humidity, temperature, or the like conditions.
A combined use of a soft polydialkylsiloxane and a rigid polyorganosilsesquioxane is expected to provide well-balanced physical properties. However, these two materials are poorly compatible with each other, and realization of such an attempt would encounter great difficulty. Along this line, JP-A-5-209031 offers a solution to the problem of compatibility between a polydialkylsiloxane and a polymethylsilsesquioxane by modifying a polydialkylsiloxane with a special acrylic resin. According to this technique, a polymethylsilsesquioxane and a polydialkylsiloxane are separately prepared as individual reactive oligomers, which are combined in a step of curing. Therefore, the method of curing and conditions therefor are naturally so limited, and yet it is difficult to obtain desired properties in a stable manner.